c - Why is malloc not "using up" the memory on my computer?

Question

So I have this program that allocates 256 MB of memory, and after the user presses ENTER it frees the memory and terminates.

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    char *p, s[2];

    p = malloc(256 * 1024 * 1024);
    if ( p == NULL) 
        exit(1);

    printf("Allocated"); 
    fgets(s, 2, stdin);
    free(p);
    return 0;
}

I ran this program multiple times and backgrounded each of them until there is no longer enough memory that can be allocated. However, that never happens. I ran a linux top command and even after running this program many times, the free memory never goes down by nearly as much as 256 MB.

However, on the other hand, if I use calloc instead of malloc then there is a HUGE difference:

p = calloc(256 * 1024 * 1024, 1);

Now if I run the program and background it, and repeat, every time I run it, the free memory goes down by 256 MB. Why is this? Why does malloc not cause the available free memory to change, but calloc does?

Answer 1

malloc() does not use memory. It allocates it.

After you allocate the memory, use it by assigning some data.

size_t Size = 256 * 1024 * 1024;
p = malloc(Size);
if (p != NULL) {
  memset(p, 123, Size);
}

Some platforms implement malloc() is such a way that the physical consumption of memory does not occur until that byte (or more likely a byte within a group or "page" of bytes) is accessed.

calloc() may or may not truly use the memory either. A system could map lots of memory to the same physical zeroed memory, at least until the data gets interesting. See Why malloc+memset is slower than calloc?